In a GaN-based LED emitting blue light, the growth substrate is typically a transparent sapphire substrate, a SiC substrate, or even a GaN substrate. For a flip chip LED, the light is generated by an active layer and exits through the transparent substrate.
FIG. 1 illustrates a conventional GaN-based flip chip LED die 10. The semiconductor layers include an N-type layer 12, an active layer 14 (forming quantum wells), and a P-type layer 16. These layers are grown on a surface of a transparent growth substrate 18, typically sapphire. On top of the substrate 18 is deposited a phosphor layer 20. Phosphor particles 22 are energized by the blue light emitted by the active layer 14 and wavelength shift the light. If the phosphor's emitted color is yellow, the combination of the yellow light and the blue light create white light. Virtually any color light may be created in this manner.
Light extraction efficiency relates to the percentage of generated photons that escape the LED die 10. One goal in designing an LED die is to minimize light absorption so as to increase light extraction efficiency. One contribution to light absorption is total internal reflection (TIR) by the substrate 18, shown by the light ray 24 being trapped inside the substrate 18, where the substrate 18 acts as a light guide. Unmatched indices of refraction at the materials' interfaces give rise to such reflections at shallow angles. As a rough approximation, the index of refraction (n) of GaN is 2.5-3, the index for sapphire is 1.77, the index for phosphor is 1.6-1.8, and the index for air is 1.
Additionally, the LED semiconductor layers, the bottom metal contacts, and the spaces between the contacts have different reflectivities. In the example shown in FIG. 1, the P-metal contacts 26, contacting the exposed P-type layer 16, are silver (Ag) so are highly reflective (>95%). In areas where the P-type layer 16 and active layer 14 are etched away to allow for ohmic contact between the N-metal contacts 28 and the N-type layer 12, a less reflective metal, such as aluminum, is used, and no light is generated over those contact regions. There are also spaces between the contacts 26 and 28 that do not reflect light. There may also be semiconductor features that also absorb light. The emitted phosphor light is generally isotropic, so a significant percentage of such light impinges on light absorbing areas of the LED die 10, such as light ray 30. Another light ray 32 is shown being internally reflected off the side of the substrate 18 and back into the LED die 10 to be partially absorbed. A light ray 33 is shown being efficiently reflected by the P-metal contact 26.
All the absorbing areas reduce the light extraction efficiency of the LED die.
What is needed is a technique for increasing the light extraction efficiency by reducing the absorption of light within an LED die.